1. Field of the Invention
The present invention relates to an implantable medical device for continuous, minimally invasive monitoring of a chemical analyte of interest using an expandable, biocompatible material incorporated into an electronic circuit component that is interrogated transdermally.
2. Description of Related Art
Various implantable medical monitoring devices have been developed to measure internal physiological conditions of a patient. For example, an implantable medical sensor that determines the oxygen content of blood using a light-emitting diode and a phototransistor is described in U.S. Pat. No. 4,815,469 to Cohen et al. U.S. Pat. No. 5,709,225 to Budgifvars et al. describes a medical implant with a capacitive sensor, which is coated with a magnetically sensitive material that causes capacitance changes in the presence of a magnetic field.
Some medical devices use sensors coupled with frequency tuned L-C circuits, where the sensor mechanically translates the changes in the physiological condition to the inductor or capacitor of the tuned L-C circuit. An external transmitter detects the resulting changes in resonant frequency of the circuit. For example, U.S. Pat. No. 5,704,352 to Tremblay et al. describes an implantable passive biosensor for monitoring physiological conditions and converting the signals to digital format. In particular, the sensors are pressure transducers that detect the pressure of cerebrospinal fluid in the cavities of a patient's brain, which is useful for monitoring the operation of a cerebrospinal fluid shunt for treating hydrocephalus. U.S. Pat. No. 4,114,606 to Seylar describes an implantable device useful for monitoring intracranial pressure for the treatment of hypertension. The resonant frequency of the passive L-C circuit implanted in the cranium varies with changes in intracranial pressure. An external monitor interrogates and detects the frequency changes in the pressure transducer. Neither of these patents describes specific mechanisms or devices for transducing other physiological conditions.
The application of a transducer in an implantable medical device that reliably monitors changes in specific chemical analytes, such as blood glucose, would be advantageous. Blood glucose levels are of particular concern because diabetes is a chronic illness that affects more than 110 million people worldwide. Conventional therapy for the most severe form of diabetes, insulin-dependent diabetes mellitus (Type I), is to administer one or more injections per day of various forms of insulin, while monitoring blood glucose levels two or three times daily with commercial glucometers that require the withdrawal of blood samples. In practice, near normal blood sugar levels are difficult to maintain with this type of therapy due to the enormous inconvenience and day-to-day burden of conventional home-monitoring techniques. The resulting large fluctuations in blood glucose levels may be responsible for a number of serious secondary ailments commonly associated with diabetes, including stroke, liver and kidney damage, and loss of eyesight.
The present invention addresses the need for a convenient, minimally invasive medical sensor that provides continuous measurements of an analyte of interest using an expandable, biocompatible material incorporated into an electronic circuit element. More specifically, this invention will help diabetic patients in monitoring blood glucose levels and achieving tighter blood glucose control without requiring blood samples to be drawn.